The present invention relates in general to humidity sensors or hygrometers and, more particularly, it relates to devices having electrical resistivity which is a function of relative humidity. Still more particularly, the invention relates to humidity sensors of the type described employing cobalt oxide.
U.S. Pat. No. 3,345,596 discloses a humidity sensor comprising a screened and fired layer of cobalt oxide on a dielectric substrate such as alumina, and with a screened and fired precious metal electrode, also including glass, on top of the cobalt oxide. To achieve hygroscopic properties the CoO must be fired at +1350.degree. C. In practice, the electrode pattern is generally in the form of closely spaced, interdigitated fingers connected at their respective bases to form two electrodes.
In co-pending U.S. application Ser. No. 443,436, now U.S. Pat. No. 3,890,703 issued June 24, 1975, and assigned to the same assignee as the instant application, it is disclosed that humidity sensors of this type are improved if the cobalt oxide is subjected to a second firing in a reducing atmosphere. In co-pending U.S. application Ser. No. 426,953, also assigned to the same assignee as the instant application, further improvements including a porous Teflon (TM) coating over the sensor, and a fired-on heating element on the back of the sensor, are described.